Heretofore, as a writing ink or jet printing ink used for recording on materials to be recorded (e.g. paper, etc.) or an ink for dyeing fibers, hides and the like, various polyazo dyes have been employed.
The polyazo dye consists mainly of a direct dye and, therefore, the dyeing process is simple and fastness is comparatively excellent. Polyazo dyes having such advantages are widely employed in the industry.
However, regarding the polyazo dye, there are problems to be solved. Firstly, there is a problem as to chemical properties, because excellent chemical properties such as stable solubility of dye, stability of concentrated dye solution with time and stability of dye density with time are required for dyes which are sued for writing inks.
Secondly, there is a problem of safety, because safety of chemical substances has recently been noted and dyes having excellent safety and which have no bad influences upon a human body are required.
As dyes wherein dyeing properties and fastness are improved, for example, benzidine or benzidine derivative based black trisazo dyes or polyazo dyes (e.g. C.I. Direct Black 19, C.I. Direct Black 154, C.I. Direct Black 168, etc.) have been widely used. However, these dyes pose question about safety for the human body because the Ames test gives false positive or positive results.
On the other hand, useful as polyazo black dyes wherein safety for humans is improved, are for example, food colors (e.g. recording solution wherein Food Black 2 is used as a recording agent, etc.), 4,4'-diaminodiphenylamine-2-sulfonic acid (substituent for benzidine) based trisazo dyes, stilbene tetrakisazo dyes having no amino group in their molecular structure and the like. However, aqueous inks wherein these dyes are used have poor water and light resistance of writing and, therefore, dyeing properties and fastness are inferior.
Polyazo dyes which satisfy the above properties have never been obtained.
Further, a water-soluble dye can be separated from aqueous systems by means such as aciding-out, salting out and the like. However, it is difficult to obtain a dye having high purity because of inclusion of an inorganic salts upon separation, whereby, various problems are caused. For example, when a dye having low purity is used for an aqueous writing ink, there is a disadvantage in that the ink is not consistently fed to a pen point. Also, when a dye of low purity is used for a jet printing ink, clogging of outlet orifice may occur.